Recently, Durnin, Miceli, and Eberly (Ref. 1) of the University of Rochester have demonstrated, both theoretically and experimentally, the existence of diffraction-free modes in free space laser beam propagation. By passing an ordinary, Gaussian profile laser beam through a thin annular slit and collimating the result, the workers generated a beam with a Jo-type profile. (`Jo` refers to the Bessel function of the zeroth order.) The advantage of the Jo profile over the Gaussian profile is that the former does not suffer geometric divergence (herein referred to as diffraction) as it propagates in free space, with no boundaries nor guiding surfaces present. Of course, plane waves also share this property. However, the Jo profile has the advantage that the beam's intensity is greatest at the center, an important feature for laser welding, laser weaponry, and laser-induced charged-particle and neutral-particle beam guidance.
Unfortunately, in the process of "reforming" the beam's profile, much of the energy is lost in absorption by the mask containing the annular slit itself. This reality severely limits the usefulness of the technique for high power applications. Additionally, although the beam does not undergo diffractive spreading, the radius of the collimating lens determines an effective maximum range.
However, this range was shown, in Ref. 1, to usually be much greater than that of the ordinary, collimated Gaussian beam. These researchers also experimentally demonstrated that the Jo-profiled beam had a greatly increased propagation range as compared to the associated Gaussian-profiled beam.
The practical difficulty with employment of this Gaussian-to-Jo coupling mechanism to problems, for instance, related to high energy, long distance laser beam transmission lies in the fact that about 99% of the laser beam's energy is absorbed by the mask: only a fraction is transmitted through the annular slit.
The system(s) detailed below all incorporate non-linear optical mechanisms. Stimulated Brillouin scattering (SBS) and two-wave mixing are described in some detail by Zel'dovich, et al (Ref. 2). Degenerate four-wave mixing (dfwm) is described in U.S. Pat. No. 4,145,671 to Hellwarth as well as Ref. 2. Both SBS and dfwm are often referred to as optical phase conjugation (opc) processes.